Since a product has been increasingly required to be thinned, in application of an optical film for a light-guiding plate, a crystal liquid display and the like for a mobile phone, a tablet PC and the like, optical defects and a poor appearance caused by minute foreign substances that has been conventionally ignored have become an issue.
A lot of techniques using a leaf-disc polymer filter are disclosed as a typical method of removing foreign substances in polycarbonate, an acrylic resin and the like. For instance, a method of manufacturing an acrylic resin has been proposed including filtration and purification of an acrylic resin composition having a melt viscosity of 2500 Pa·sec or less at 260 degrees C. using a leaf-disc filter of a filtration accuracy of 3 to 25 μm (Patent Literature 1).
Moreover, a method of pelletizing a polymer melt of polycarbonate, which is manufactured by ester interchange, through a leaf-disc polymer filter has been known. For instance, a method of preventing generation of gels by increasing a temperature of the leaf-disc polymer filter with inert gas (e.g., nitrogen) has been proposed (Patent Literature 2).
Further, as the method of removing foreign substances in polycarbonate, for instance, a method has been proposed including polymerizing polycarbonate, adding an additive in a melt state of the polymerized polycarbonate, filtrating the obtained mixture with a polymer filter having an absolute filtration rating in a range from 0.5 to 50 μm, and kneading the filtrated mixture (Patent Literature 3).
Moreover, a method of decreasing volatile impurities by melting and deaerating polycarbonate is also known. For instance, one of the proposed methods of decreasing volatile impurities includes: disposing a biaxial extruder, a polymer filter and a vent biaxial extruder in series; passing polycarbonate through the polymer filter; and melting and deaearting the polycarbonate with the vent biaxial extruder (Patent Literature 4).